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As a core transmission component in industrial automation, CNC machine tools, and precision equipment, ball screws are renowned for their high efficiency, high precision, and long service life. They realize the conversion between rotary motion and linear motion with minimal energy loss, becoming a key guarantee for the precision and stability of mechanical systems. This article will systematically interpret the professional knowledge of ball screws from the perspectives of definition, structural composition, scientific classification, precision grading, technical characteristics, and practical selection, helping you deeply grasp the technical essence of this critical component.
1. Accurate Definition & Core Working Principle of Ball Screws
A ball screw (also known as a ball bearing screw) is a mechanical transmission device that uses high-precision steel balls as rolling elements between the screw shaft and the nut to convert the rotary motion of the screw into the linear motion of the nut (or vice versa). Compared with traditional trapezoidal screws that rely on sliding friction, the rolling friction between steel balls and the raceway of the screw/nut greatly reduces the friction coefficient, enabling higher transmission efficiency and positioning accuracy.
Core working principle: When the screw shaft rotates under the drive of a power source (such as a servo motor), the steel balls in the nut raceway roll along the spiral groove of the screw. Under the constraint of the return device (ball circulation system), the steel balls circulate continuously between the screw and the nut, avoiding mutual collision and wear. While the steel balls roll, they drive the nut to perform linear motion along the axis of the screw; conversely, when the nut is subjected to linear force, it can drive the screw to rotate, realizing bidirectional conversion between rotary and linear motion.
2. Core Structural Composition of Ball Screws
A complete ball screw assembly consists of five key components, each of which directly affects the transmission performance and service life of the product. The rationality of the structure design is the basis for ensuring high precision and high efficiency:
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Screw Shaft: The core component with a spiral raceway processed on the surface, usually made of high-carbon chromium bearing steel (SUJ2) or alloy structural steel (SCM440). After quenching and tempering, precision grinding, and other processes, it has high hardness (HRC58-62) and surface finish (Ra ≤ 0.2 μm), ensuring the smooth rolling of steel balls and wear resistance.
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Nut: The matching part with the screw shaft, with a spiral raceway corresponding to the screw inside. The material is the same as the screw shaft, and the raceway is subjected to precision grinding to ensure the consistency of the fit with the screw. The nut is also equipped with a mounting interface for connecting with the linear motion component (such as a workbench).
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Steel Balls: The rolling elements between the screw and the nut, usually made of high-precision bearing steel balls (G10-G3), with a diameter tolerance of ±0.001mm. The size and number of steel balls directly determine the load-bearing capacity and rigidity of the ball screw.
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Return Device (Ball Circulation System): The key component to realize the continuous circulation of steel balls, which can be divided into internal circulation and external circulation according to the circulation mode. Its function is to guide the steel balls that have rolled to the end of the nut back to the starting end of the raceway, ensuring the continuity of transmission. The design of the return device directly affects the running smoothness and noise level of the ball screw.
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Sealing Device: Installed at both ends of the nut and the outer circumference of the nut, it is used to prevent dust, metal chips, cutting fluid, and other impurities from entering the raceway, and at the same time prevent the leakage of lubricating oil. Common sealing forms include contact seals (such as felt rings, rubber seals) and non-contact seals (such as labyrinth seals), which are selected according to the working environment.
3. Scientific Classification of Ball Screws
Ball screws can be divided into different types according to multiple professional dimensions. Clarifying the classification criteria helps to accurately match the product to the application scenario. The main classification methods in the industry are as follows:
3.1 Classification by Ball Circulation Mode
This is the most common classification method, which is divided according to the way the steel balls circulate in the nut:
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Internal Circulation Ball Screw: The steel balls circulate inside the nut. The return device is a reverse channel processed inside the nut (usually a circular arc groove or a through hole). The steel balls enter the reverse channel from the end of the raceway and return to the starting end. Advantages: Compact structure, small nut volume, high running smoothness, low noise (usually ≤ 60dB), and suitable for high-speed operation (maximum speed up to 3000 rpm). Disadvantages: Complex processing technology and relatively high cost. Common in CNC machine tools, precision electronic equipment, and other high-precision scenarios.
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External Circulation Ball Screw: The steel balls circulate outside the nut. The return device is a steel pipe or a plastic guide groove installed on the outer surface of the nut. The steel balls roll out of the nut raceway, enter the return pipe, and return to the other end of the nut. Advantages: Simple processing technology, low cost, easy maintenance, and can be designed with multiple circuits to increase the number of steel balls and improve load-bearing capacity. Disadvantages: The nut volume is large, the running noise is relatively high, and the maximum speed is limited (usually ≤ 2000 rpm). Suitable for general automation equipment, heavy machinery, and other scenarios with low requirements for speed and noise.
3.2 Classification by Screw Groove Profile
Divided according to the cross-sectional shape of the spiral raceway on the screw and nut, which affects the contact state between the steel ball and the raceway:
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Arc Groove Ball Screw: The raceway cross-section is an arc shape with a radius slightly larger than the steel ball radius (usually 1.02-1.05 times the steel ball radius). Advantages: Good contact stability, strong ability to bear radial loads and overturning moments, and high rigidity. Disadvantages: The contact area between the steel ball and the raceway is small, and the load-bearing capacity is relatively limited. Suitable for high-precision positioning scenarios with small loads.
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Gothic Arch Groove Ball Screw: The raceway cross-section is a Gothic arch shape (composed of two arcs with opposite radii). Advantages: The steel ball is in contact with the raceway at two points, which can bear both axial loads and radial loads, and the load-bearing capacity is 1.5-2 times that of the arc groove. Disadvantages: High processing precision requirements, and the contact state is sensitive to installation errors. Suitable for heavy-load, high-rigidity scenarios such as heavy-duty CNC machine tools and hydraulic presses.
3.3 Classification by Lead Accuracy
Divided according to the lead error (the deviation between the actual lead and the theoretical lead), which is the core index reflecting the positioning accuracy of the ball screw. The classification standards refer to international standards (ISO 3408) and national standards (GB/T 17587.1-2017):
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Precision Grade C1-C5 (High Precision): Lead error is small (C1 grade lead error ≤ 0.003mm/300mm, C5 grade ≤ 0.012mm/300mm), with high repeat positioning accuracy (≤ 0.005mm). After precision grinding and fine adjustment, it is suitable for ultra-precision equipment such as semiconductor packaging machines, optical instrument positioning stages, and precision CNC machining centers.
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Precision Grade C7-C10 (Medium Precision): Lead error is moderate (C7 grade ≤ 0.025mm/300mm, C10 grade ≤ 0.050mm/300mm), which balances precision and cost. It is the most widely used grade in industrial automation, suitable for general CNC machine tools, linear modules, robot arms, and other equipment.
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Precision Grade C16 (General Precision): Lead error is relatively large (≤ 0.100mm/300mm), processed by rolling forming, with high production efficiency and low cost. Suitable for low-precision transmission scenarios such as ordinary conveyors, automatic door mechanisms, and simple lifting platforms.
3.4 Classification by Installation Form
Divided according to the fixed form of the two ends of the screw shaft, which affects the rigidity and stroke of the ball screw:
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Fixed-Fixed Type: Both ends of the screw are fixed with angular contact ball bearings. Advantages: The highest rigidity, can bear large axial loads and overturning moments, and the critical speed is high, suitable for long-stroke, high-speed, and high-rigidity scenarios (such as large-scale CNC machine tools).
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Fixed-Free Type: One end of the screw is fixed, and the other end is free (no bearing constraint). Advantages: Simple installation, can compensate for thermal expansion and contraction of the screw during operation. Disadvantages: Low rigidity, limited load-bearing capacity, suitable for short-stroke, low-speed scenarios (such as small electronic equipment).
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Fixed-Supported Type: One end of the screw is fixed, and the other end is supported by a deep groove ball bearing. Advantages: Balances rigidity and installation difficulty, can bear a certain axial load, and is suitable for medium-stroke, medium-speed scenarios (such as general automation modules).
4. Core Technical Indicators of Ball Screws
Understanding the core technical indicators is the key to evaluating the performance of ball screws and selecting products. The main technical indicators include the following aspects:
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Lead (P): The linear distance that the nut moves axially when the screw rotates one full turn (360°), in millimeters (mm). The lead directly determines the transmission speed (linear speed = lead × rotational speed) and positioning resolution. Common leads include 5mm, 10mm, 20mm, etc. Fine leads (≤ 5mm) are suitable for high-precision positioning, and coarse leads (≥ 20mm) are suitable for high-speed transmission.
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Lead Accuracy: As mentioned earlier, it is divided into C1-C16 grades, which is the core index of positioning accuracy. When selecting, it is necessary to match the precision grade according to the actual positioning requirements of the equipment.
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Axial Load Capacity: The maximum axial force that the ball screw can bear during operation, in Newtons (N). It is determined by the size of the steel ball, the number of steel balls, and the raceway profile. Exceeding the load capacity will lead to premature wear and reduced precision.
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Rigidity: The ability to resist deformation under load, including axial rigidity and radial rigidity. Axial rigidity is particularly important for positioning accuracy, and it can be improved by increasing the diameter of the screw, selecting a reasonable installation form, or preloading.
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Critical Speed: The maximum rotational speed at which the screw does not produce resonance during operation, in revolutions per minute (rpm). Exceeding the critical speed will cause violent vibration of the screw, affecting transmission stability. The critical speed is related to the diameter, length, and installation form of the screw.
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Transmission Efficiency: The ratio of output power to input power, which is as high as 90%-98% for ball screws (only 30%-50% for trapezoidal screws). High efficiency means less energy loss, which is conducive to energy saving and reducing the load of the drive motor.
5. Practical Selection Guide for Ball Screws
The correct selection of ball screws directly affects the performance, service life, and cost of the equipment. It is necessary to comprehensively consider the following factors and avoid blind pursuit of high precision or low cost:
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Clarify Application Requirements: First, determine the core requirements of the equipment, including positioning accuracy (determining the precision grade), load capacity (determining the screw diameter and lead), running speed (determining the lead and critical speed), and stroke (determining the screw length and installation form).
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Select the Appropriate Circulation Mode: For high-speed, low-noise, and small-space scenarios, select internal circulation ball screws; for general-load, low-cost scenarios, select external circulation ball screws.
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Match the Precision Grade: For ultra-precision equipment such as semiconductor and optical instruments, select C1-C5 high-precision grades; for general CNC machine tools and automation equipment, select C7-C10 medium-precision grades; for low-precision transmission, select C16 general-precision grades.
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Determine the Installation Form: For long-stroke, high-rigidity requirements, select fixed-fixed type; for short-stroke, simple installation requirements, select fixed-free type; for medium-stroke, balanced rigidity and installation difficulty, select fixed-supported type.
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Consider the Working Environment: In dusty, humid, or corrosive environments, select ball screws with enhanced sealing devices (such as labyrinth seals) and surface anti-corrosion treatment (such as nickel plating, chrome plating); in high-temperature environments, select high-temperature resistant materials and lubricants.
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Pay Attention to Preloading Requirements: For scenarios requiring high rigidity and no backlash (such as CNC machining centers), select preloaded ball screws (common preloading methods include double nut preloading, offset preloading, and lead preloading); for general scenarios, non-preloaded ball screws can be selected to reduce cost.
6. Daily Maintenance and Service Life Extension Tips
Proper daily maintenance can effectively extend the service life of ball screws and maintain their precision. The key maintenance points are as follows:
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Regular Lubrication: Lubricating oil or grease should be added regularly to reduce the friction between steel balls and raceways. The type of lubricant should be selected according to the working speed and temperature (high-speed scenarios use lubricating oil, low-speed heavy-load scenarios use grease). It is recommended to lubricate every 200-500 hours of operation.
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Sealing and Dust Prevention: Check the sealing device regularly to ensure that it is intact and effective. Clean the surface of the screw and nut in time to prevent impurities from entering the raceway. For harsh environments, additional protective covers (such as telescopic covers) can be installed.
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Regular Precision Inspection: Use tools such as dial indicators and laser interferometers to check the positioning accuracy and lead error of the ball screw regularly. If the precision exceeds the allowable range, adjust or replace the ball screw in time.
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Avoid Overload Operation: Strictly control the load and speed within the rated range of the ball screw to avoid premature wear or damage caused by overload and overspeed.
Conclusion
Ball screws, as the "precision core" of mechanical transmission, play an irreplaceable role in industrial automation and high-precision equipment. From accurate definition and structural composition to scientific classification and precision grading, every link reflects professional technical connotations. When selecting and applying ball screws, it is necessary to comprehensively consider application requirements, working environment, and cost factors, and match products with appropriate specifications and performance. At the same time, standardized daily maintenance can ensure the long-term stable operation of ball screws.
For engineers and technical personnel engaged in mechanical design and automation, in-depth grasp of ball screw knowledge is the basis for improving equipment performance and reducing failure rates. With the continuous development of industrial automation, ball screws will move towards higher precision, higher speed, and more environmental adaptability, providing stronger support for the intelligent upgrading of the manufacturing industry.
Table of Contents
- 1. Accurate Definition & Core Working Principle of Ball Screws
- 2. Core Structural Composition of Ball Screws
- 3. Scientific Classification of Ball Screws
- 4. Core Technical Indicators of Ball Screws
- 5. Practical Selection Guide for Ball Screws
- 6. Daily Maintenance and Service Life Extension Tips
- Conclusion